1. Field of the Invention
The present invention relates to a control device for an automatic transmission, and, in particular, to a control device for an automatic transmission which performs a neutral control operation.
2. Description of the Related Art
In general, in a control device for an automatic transmission, when the vehicle is stopped in a forward range, a creep phenomenon occurs in which the driving force generated by the idling rotation of the engine is transmitted as creep force to the driving wheels via torque converter and transmission mechanism. To keep the vehicle stationary, the driver must operate the brake to suppress the creep force, and the fuel efficiency of the engine decreases by the amount corresponding to the creep force consumed by the brake. Thus, when vehicle is stationary in a forward range with the brake pedal depressed to actuate the brake and accelerator almost fully closed, the control device for an automatic transmission performs a neutral control operation to shift the automatic transmission from a drive mode to a neutral mode in which the forward clutch is half-engaged and slips with the vehicle in the forward range.
A control device for an automatic transmission of this type decreases the engaging pressure on the forward clutch to decrease the temperature of the forward clutch to a predetermined value or lower so that the forward clutch cannot be excessively heated by the frictional heat caused by the slippage during the neutral control operation (see Japanese Patent Application Publication No. 2007-9930 (JP-A-2007-9930), for example).
While performing a neutral control operation, the control device for an automatic transmission disclosed in JP-A-2007-9930 estimates the temperature of the forward clutch based on the engine speed, the ratio between the engine speed and the turbine speed of the torque converter, the hydraulic oil temperature and the duration of the neutral control operation, and decreases the engaging pressure on the forward clutch to reduce generation of frictional heat when the estimated forward clutch temperature exceeds a predetermined temperature.
However, in the above control device for an automatic transmission, when the temperature of the hydraulic oil for use in engaging the forward clutch is low, the response of hydraulic pressure is slow, in other words, the actual hydraulic pressure does not follow a desired hydraulic pressure, because of the high viscosity of the hydraulic oil, and the engaging pressure on the forward clutch may not rise as desired when the automatic transmission is returned from the neutral mode to the drive mode. Also, if the engaging pressure on the forward clutch is decreased in view of the fact that the viscosity of the lubricating oil supplied to the frictional engagement element is also high when the temperature is low, the solenoid for controlling the engagement and disengagement of the forward clutch operates in a low pressure range in which the pressure response is slow and the delay in the rise of the engaging pressure on the forward clutch may increase when the automatic transmission is returned from the neutral mode to the drive mode.
As described above, when the temperature of the hydraulic oil is low, the rise in the engaging pressure on the forward clutch delays because of the slow response of the hydraulic pressure or some other reason when the automatic transmission is returned from the neutral mode to the drive mode. Thus, the forward clutch cannot be engaged smoothly when the vehicle is started. As a result, a shock occurs to the vehicle or the starting performance of the vehicle is deteriorated, resulting in poor driveability.